3 Answers
3

The name "John Doe" is used in several English-speaking countries to refer to a person whose actual identity is unknown. You may hear this in police reports, hospital admissions, forensic examinations, or any other context in which officials need to refer to a person whose actual name they don't know. John Doe (and its feminine equivalent Jane Doe) are effectively "placeholder names" used when a name is required and nothing else is available.

This is not unique to the 1940's, and as far as I know the usage has not changed since that time.

Looking for a John Doe in the Corpus of Historical American I notice that the phrase was used before the 1940s in sentences like the following ones.

Mr. Craig could be spared from his office as readily as Mayor Hylan has been, nor would his family be left in want as in the case of many a poor devil of a John Doe.
The proceedings before the grand jury will be in the nature of a John Doe action involving a possible indictment for felony under the law forbidding the granting or obtaining of public office in exchange for a gift, promise or other compensation.

The CoHA didn't report any sentences containing a John Doe dated 1940s, but it reports sentences like this:

Filipino health was about, the best in the Orient: in 35 years, cholera, smallpox and bubonic plague had been wiped out; the population had increased from seven to 16 million, and the average height of the "tao" (John Doe) from 4 ft. 11 in. to 5 ft. 4 in.